Known picture-in-picture television receivers include two video signal channels, a main and an auxiliary channel, each including a tuner; an IF chain; and a video detector. The information from the auxiliary channel is compressed and stored in a memory in synchronism with the auxiliary video signal. This stored information is retrieved in synchronism with the main video signal and replaces a portion of the main video signal at a predetermined image location. In this way, a picture-in-picture video signal is formed representing an image having a first region which displays the main video image, as represented by the main video signal, and a second, usually smaller, region which displays the auxiliary video image, as represented by the auxiliary video signal.
An NTSC standard video signal consists of successive frames of 525 lines made up of two interlaced fields of 2621/2 lines each. A sampled-data processed NTSC signal sampled at a rate of four times the color subcarrier frequency contains 910 samples in each line. The auxiliary video signal is compressed by, for example, sampling it at the above rate, and storing in the memory only every third sample of every third line. Each field of compressed auxiliary image information, thus, comprises 87 lines of 303 samples each.
In each NTSC video field, 21 lines form a vertical blanking interval (VBI) and do not contain image information. Thus, in general, seven lines (1/3 of 21) of the compressed auxiliary video signal contain VBI signal information and need not be displayed in the inserted auxiliary image. Only the 80 remaining lines contain image information. In addition, in each line, approximately 150 samples form a horizontal blanking interval (HBI) and do not contain image information. Thus, in general, 50 samples (1/3 of 150) of compressed auxiliary video signal contain HBI signal information and need not be displayed in the inserted auxiliary image. Only the 253 remaining samples contain image information.
In each field of a sampled data main video signal, a portion, consisting of 253 adjoining samples of 80 adjoining lines, is replaced by the previously stored non-VBI and HBI compressed auxiliary samples. If this portion is located in the lower right hand corner, for example, samples 607 through 859 (totaling 253 samples) of lines 182 through 261 (totaling 80 lines) of the main video signal may be replaced with the previously stored compressed auxiliary video samples to form the picture-in-picture video signal. Alternatively, the previously stored 80 lines of 253 samples each, which represent the auxiliary image, may be retrieved from the memory, converted into a continuous signal and substituted into a corresponding portion of a main video signal, which is also in continuous form.
The memory may be envisioned as being subdivided into three blocks, each capable of storing one field of compressed auxiliary video information. Successive fields of compressed auxiliary video information are written into these blocks in round robin fashion. Fields of previously stored compressed auxiliary video information are retrieved from the blocks, also in round robin fashion, so that no block is written into and read from simultaneously.
Dual port memories have recently become available which have a high memory capacity (for example, they are capable of storing a full field of video information), and are priced such that integration into a consumer television receiver is feasible. For example, the HM 53051P, 262,144-word 4-bit frame memory, manufactured by Hitachi, is such a dual port video memory system. Such a high capacity memory chip allows flexibility in use which was previously unavailable with lower capacity memory integrated circuits.
The HM 53031P includes a data input terminal and a write address terminal. Standard random access memories (RAM's) require each sample to be stored to be accompanied by an address signal. Unlike the standard RAMs, an address signal need not be supplied concurrently with each sample to be stored for the HM 53031P. Instead, the write address terminal receives an initial write address. Subsequent successive samples are sequentially stored in storage locations beginning at the location corresponding to the last received initial write address. The HM 53031P further includes a data output terminal and a read address terminal. The read address terminal receives an initial read address. Samples are retrieved from sequential locations in the memory beginning at the location corresponding to the last received initial read address. In the remainder of this specification, such a memory will be referred to as a self-sequencing memory. It is desirable to incorporate such a memory in a picture-in-picture video signal generator.